1. Field of the Invention
The present invention is related to printed circuit board testing, and more specifically, to the measurement and testing of a printed circuit board's controlled impedance.
2. Description of the Related Art
As the value and use of information continues to increase, individuals and businesses seek additional ways to process and store information. One option available to users is information handling systems. An information handling system generally processes, compiles, stores, and/or communicates information or data for business, personal, or other purposes, thereby allowing users to take advantage of the value of the information. Because technology and information handling needs and requirements vary between different users or applications, information handling systems may also vary regarding what information is handled, how the information is handled, how much information is processed, stored, or communicated, and how quickly and efficiently the information may be processed, stored, or communicated. The variations in information handling systems allow for information handling systems to be general or configured for a specific user or specific use such as financial transaction processing, airline reservations, enterprise data storage, or global communications. In addition, information handling systems may include a variety of hardware and software components that may be configured to process, store, and communicate information and may include one or more computer systems, data storage systems, and networking systems.
Information handling systems such as computers and associated storage disk arrays use high speed data and control buses comprising a plurality of printed circuit traces or conductors on a printed circuit board (PCB) over which high speed (fast rise and fall time) signals, e.g., electrical pulses, travel between the electronic circuits, e.g., microprocessor, memory, programmable logic array (PLA), application specific integrated circuit (ASIC), serial and parallel interfaces and the like, mounted on the printed circuit board. It is important that these printed circuit traces have a controlled impedance so as to minimize degradation of the high speed signals, e.g., noise, reflections, crosstalk, skewing and the like. Therefore it is important to verify the quality of PCB construction by testing and measuring the impedance of the printed circuit board traces, and the quality and uniformity of the dielectric value of the material comprising the insulated portion of the PCB.
Impedance coupons are used to check the quality of PCB construction. For high performance PCB's, it is necessary for the electrical impedance to be controlled, such that all data buses function within specification. Impedance of individual PCB traces can vary depending on many factors, e.g., trace width and thickness, dielectric constant of the PCB material. As a quality inspection tool, impedance coupons may be added to the PCB design. The impedance coupons may be conductive traces on the PCB of a approximately 6 inches in length and may be adapted where a time domain reflectometer can be used to measure the impedance of the conductive traces. Being able to easily and reliably measure the impedance coupon can directly affect determining the yield of the PCB, thus better impedance measurement techniques help ensure cost effective production of the high performance PCBs without unnecessary waste and error by miss-measuring good PCBs or erroneously accepting defective PCBs.
The PCB must be tested by measuring the controlled impedance to determine if the required controlled impedance specification is met. However, using standard test and measurement equipment, it can be difficult to assess where to begin or end the measurement of an impedance coupon.